Bug of the socket output, is not it?
This is a missing feature of the node API. There is a patch for this ready and waiting, but since it changes the way node sockets look i want to wait for green light from scripters before committing to trunk.

See bf-committers thread

UPDATE: This feature has been added in trunk now (r60661). Socket draw functions are responsible for drawing only labels if that is the desired behavior, otherwise inputs and outputs draw the same way. See the API changes wiki page.